Keith,
I am just going through the getting ready stage to build a new engine, and that can run on either gas (propane, butane or a butane/propane mix) or petrol. A lot of the engines we make, hit & miss and other types of engines like them should be more than happy running on gas. I don't know about the higher speed ones or multi cylinder.
The main difference is the carb, the gas one is much easier to make, just basically a tube with regulated gas fed into it with a butterfly to control the speed range.
I have attached at the bottom a very low res picture from the plans of my engine, and it should show you how easy it would be to make.
One thing you must consider though, you need to control the gas before it gets to the engine.
On the gas tank, it should have an on/off valve with a built in pressure regulator, and between the tank and carb, ideally you should fit a demand valve. This only allows gas to the engine while the engine requires it, if the engine stops, the demand valve cuts off the gas supply. It is really a safety measure, to prevent explosions if the engine stops, if it isn't there, the gas can continue to be fed even though the engine isn't running. They are not difficult to make at all.
I hope this is of help
John
